What are Calculate Parity reads (CP reads) and why it makes extra disk reads?

Applies to

• ONTAP 9
• Data ONTAP 8
• 7-mode

Answer

• Disk cpreads are blocks that are read by RAID to satisfy a write known as Calculate Parity Reads
• Consistency Point are a normal state of Data ONTAP processing under which dirty buffers are de-staged to disk
  • Writes are buffered in RAM until written to disk by the Consistency Point process
• The Consistency Point process requires successful pre-fetch of metadata blocks in order to write the data to disks, where cpreads come in.
• From statit, cpreads column refers to CP reads (Calculate Parity Reads) these are extra reads apart from user reads which can also contribute to Disk reads

disk             ut%  xfers  ureads--chain-usecs writes--chain-usecs cpreads-chain-usecs

/aggr1/plex0/rg0:

9d.01.0           36  99.01    0.00   ....     .  47.50  62.96   179  51.51  57.25    81 

9d.02.0           34 100.39    0.00   ....     .  48.92  61.20   162  51.48  57.25    68 

1c.03.0           56 206.74   99.04   3.22  2035  40.75  40.53   315  66.95  14.92   338 

9d.00.0           73 206.66  100.15   3.22  2993  40.26  40.76  1004  66.25  14.80   640 

9d.01.1           73 208.33  101.16   3.31  2918  40.38  40.48  1021  66.79  14.91   643 

Additional Information

NA